Implantable pacemakers generate electrical stimulation pulses and deliver such stimulation pulses to atrial and/or ventricular muscle tissue of a patient's heart at a prescribed rate and/or rhythm when, through disease or other cause, the heart is not able to maintain the prescribed heart rate or rhythm on its own. When the delivered electrical stimuli are of sufficient energy, they cause the cardiac muscle tissue to depolarize, and therefore contract, thereby forcing the heart rate or rhythm to track the delivery of the electrical stimuli. When the delivered electrical stimuli are of insufficient energy, depolarization does not occur, and the heart rate or rhythm is not controlled by the pacemaker. Hence, for the pacemaker to perform its intended function, it is important that the delivered electrical stimulus is of sufficient energy to depolarize the cardiac tissue.
The depolarization and ensuing contraction of the heart in response to a delivered cardiac stimulation pulse is generally referred to in the art as “capture”. Consequently, the term “non-capture” denotes the condition when a delivered stimulation pulse does not result in depolarization and contraction. When detecting capture, sensing circuitry checks for the depolarization of a cardiac chamber following and in response to a delivered stimulation pulse. Such a depolarization as a result of a delivered stimulation pulse is also referred to as an “evoked response” (ER) of that chamber. Furthermore, the evoked response is detected during a selected time period following the delivery of a stimulation pulse. Such a time period is generally referred to as an “evoked response window”.
The amount of energy needed to effectuate capture is known as the capture “threshold”, and electrical stimuli of energy less than the capture threshold do not bring about capture, while electrical stimuli of energy greater than the capture threshold do bring about capture. A capture threshold search normally begins at a desired starting point (either a high energy level or the level at which capture is currently occurring) and the energy level is decreased until capture is lost. The smallest value at which capture is maintained is known as the capture threshold. Thereafter, in order to secure capture, a safety margin is added to the capture threshold to arrive at the energy content of the stimulation pulse.
At least some existing approaches for capture management threshold testing rely on the availability and use of two separate pacing output capacitors and associated charging circuitry. In general, a first output capacitor provides a primary pacing stimulus at a “test” amplitude and duration, and a second output capacitor provides a secondary pacing stimulus at a “backup” amplitude and duration. The backup pulse is generally delivered between approximately 50 milliseconds (ms) and 250 ms after the test pulse.
This two-capacitor approach is commonly employed because the relatively short interval between the test and backup pulses makes it essentially impractical to recharge a single tank capacitor from a test voltage level (which is generally relatively low) to a backup voltage level (which is generally relatively high) within the required timeframe.